Studies of alkali-silica reaction. Part 7. Modelling of expansion

CEMENT and CONCRETE RESEARCH. Vol. 20, pp. 285-290, 1990. Printed in the USA. 0008-8846/90. $3.00+00. Copyright (c) 1990 Pergamon Press pl¢.

STUDIES OF A L K A L I - S I L I C A REACTION. PART 7. MODELLING OF EXPANSION S. Chatterji and Teknologisk Inst. 2630 Taastrup Denmark

P. Christensen PC Laboratoriet A/S M~gevej 7 9690 Fjerritslev Denmark

(Refereed) (Received June 21; in final form Sept. 20, 1989)

ABSTRACT It has been shown that expansion of mortar prisms, due to a l k a l i - s i l i c a reaction, can be q u a n t i t a t i v e l y related to their environment, i.e. alkali ion concentration and temperature by an equation of the type __. d21 dl dl AT + - -AC.AT) Al(o/oo) = S-K(1 - ~-~-AC - dT dC-dT The physical significance of negative signs of dl/dC and dl/dT terms have been discussed. This relation permits a rational choice of aggregates for a given environment. Introduction In recent years we have carried out an extensive series of work on a l k a l i - s i l i c a reaction and its associated expansion. It has been shown that NaCI accelerates alkali-silica expansion and the presence of Ca(OH) 2 is a prerequisite of this expansion (I, 2). It has also been shown that at the testing temperature of 50oc the expansion decreases with decreasing concentration of NaCI solution (3). Until now no quantitative relationship has been drawn between expansion and the factors which affect the expansion; only exception being a relationship relating expansion and the air-content of the mortar prisms (4). In this paper we shall report the work done to relate quantitatively the expansion of prisms to temperature and NaCI concentration of their environment. Experimental Technique A large number of 40x40x160 mm mortar prisms were cast from each of the two sand types, i.e. Nymolle and Kallerup used in this investigation; in each case the mortar had the composition of I part low alkali Portland cement, 3 parts sand, and 0.5 part water all by weight. Air contents of the mortars were kept 285

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Vol. 20, No. 2 S.

Chatterji

and

P.

Christensen

Nymslle sand, 70 C Legend: 1-1N+O.SN 2-2N

S-eat.

018 o

010 C 0

,s._6------e....... s--s....... s .............. s ................ s ........ s

c'- 5

Q.

i ,2----~-----~-z----~ .........~ ..........-2------2

X

i 1

0 0

10

20

30

1 40

1 50

1 60

70

1 80

90

Time in weeks Fig.

I.

Time

expansion

curve.

Nymlalle sand, 50 C Legend: 1.1N 2-2N

S=SaL

below 4% by volume. After 27 days water curing the prisms of each series were d i v i d e d into g r o u p s of 3. Each g r o u p was then placed in a container c o n t a i n i n g sodium c h l o r i d e s o l u t i o n of either s a t u r a t e d or 2 or I or 0.5 normal strength. The c o n t a i n e r s were then t i g h t l y stoppered and placed in a bath maintained either at 70 or 50 or 30 or 20oc. All together 26 g r o u p s of prisms were studied. At intervals the lengths of the prisms were m e a s u r e d and at the same time the s o l u t i o n s were rep l a c e d by new solutions. The length m e a s u r e m e n t was continued up to 82 weeks.

o18i 0

Results

01o ¢-

The time vs e x p a n s i o n curves for the two sand types are shown in F i g u r e s I to 8. An e x a m i n a t i o n of the above figures will show the following general characteristics:

9_ i,,- 5

:' ~

o. X U.l

1

1

1

1

o o

10

20

30

40

50

60

70

80

90

a) Each group of prisms showed an initial period of slow e x p a n s i o n followed by a period of fast e x p a n s i o n and then a period of nearly no expansion. The length of the initial slow e x p a n s i o n period increased with d e c r e a s i n g t e m p e r a t u r e . This happened i r r e s p e c t i v e of the sand type or salt c o n c e n t r a t i o n .

Time in weeks Fig.

2.

Time

expansion

curve.

Nym~lle sand, 30 C Legend: 1-1N 2-2N

S-SaL

0 15 o

b) For any sand and salt conc e n t r a t i o n the e x p a n s i o n increased with the d e c r e a s i n g t e m p e r a t u r e of storage.

e-

.£

¢n r- 5 Q.

. ' _ _ - - - s - ~ - - - s

................ s ........ s

x

w

o o

lO

20

30

40

50

60

70

80

Time in weeks Fig.

3.

Time

expansion

curve.

and D i s c u s s i o n s

90

c) Most i n t e r e s t i n g l y the c o n d i t i o n s for the m a x i m u m e x p a n s i o n for any one sand were 20°C and I normal salt solution. This is best seen in Figures 4 and 8.

Vol. 20, No. 2

287 ALKALI-SILICA REACTION, EXPANSION MODELING

Nym~lle sand, 20 C Legend: 1=1N 2=2N S-SaL

0

o~ .o

S J/.

f'5

s

/

,/"

/

x o 0

10

20

30

40

50

60

70

80

90

Time in weeks Fig.

4.

Time

expansion

Q u a l i t a t i v e l y it is not very d i f f i c u l t to u n d e r s t a n d why lowering the t e m p e r a t u r e of storage increased the length of the initial slow e x p a n s i o n period; this might have been due to slow p e n e t r a t i o n of s o d i u m salt in the prisms and lowering of the rate of a l k a l i - s i l i c a reaction. However, at p r e s e n t no m a t h e m a tical e x p r e s s i o n could be f o r m u l a t e d w h i c h could predict the time vs e x p a n s i o n curves.

curve.

To our s a t i s f a c t i o n we could f o r m u l a t e m a t h e m a t i c a l e x p r e s s i o n s relating the m a x i m u m e x p a n s i o n s shown by g r o u p s of prisms to their storage c o n d i t i o n s i.e. salt c o n c e n t r a t i o n and the temperature. The g e n e r a l m a t h e m a t i c a l e x p r e s s i o n has the form: A1(o/oo)=SxKx(1-Mx(C-co)-Nx(T-to)+Px((C-co)x(T-to))

......

(A)

where AI

is the u l t i m a t e

expansion

of tne group

S, K, M, N, P are e x p e r i m e n t a l C, T°C are the salt age bath

constants

concentration

and

temperature

of the stor-

Co, t o are the c r i t i c a l c o n c e n t r a t i o n and t e m p e r a t u r e below w h i c h no a l k a l i - s i l i c a e x p a n s i o n occurs. From our p r e v i o u s work (3) and the l i t e r a t u r e (5) 'c o ' was found to be 0.5 normal and 't o ' to be 0°C. The e q u a t i o n best is:

which

fitted

the e x p a n s i o n

data of N y m ® l l e

A1(o/oo)=16.47x(1-0.096x(C-0.5)-0.014xT+0.0022x(C-0.5)xT) The c o r r e s p o n d i n g

equation

for K a l l e r u p

sand

nearly e q u a l l y good fit for both sets of d a t a with the following common equation:

AI(o/oo)=SxKx(1-0.06x(C-0.5)-0.0142xT+0.0015x(C-0.5)xT) where SXK = 16 for N y m o l l e

sand

and 8.1

.. (B)

is:

A1(o/oo)=8.13x(1-0.053x(C-0.5)-0.0144xT+0.0011x(C-0.5)xT) However, obtained

sand

for K a l l e r u p

sand.

.. (D) could

....

be

(E)

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S. Chatterji and P. Christensen

It is of interest more closely:

to e x a m i n e

the nature

of the e q u a t i o n s

A to E

(i) They show that the m a x i m u m e x p a n s i o n c a p a c i t y of a sand is m a i n l y d e t e r m i n e d by the p r o d u c t SxK. It is known from the literature that the e x p a n s i o n c a p a c i t y of a sand is d i r e c t l y related to its r e a c t i v e silica content. The c o n s t a n t S could then be identified to the volume fraction of the r e a c t i v e silica in the mortar; in that case K r e p r e s e n t s a p r o p o r t i o n a l i t y constant relating reactive silica to expanison. The p r o p o r t i o n a l i ty c o n s t a n t K also i n c o r p o r a t e s the effect of the p a r t i c l e size d i s t r i b u t i o n of the reactive grains. K tends to zero as size of r e a c t i v e silica tends to the cement fineness or as Ca ++ ion c o n c e n t r a t i o n in the liquid phase tends to zero. (ii) The c o n s t a n t s M and N could be c o n s i d e r e d as dl/dC and dl/dT; in that case they are expected to be i n d e p e n d e n t of sand types. (iii) The c o n s t a n t P could be c o n s i d e r e d as d21/dC-dT; from the e q u a t i o n E the n u m e r i c a l value of P is found to be 2-dl/dC.dl/dT. In this case the e q u a t i o n E could be w r i t t e n as Al(o/oo) (iv)

dl dl d21 - ~-~.AC - ~-~.AT + dC.d-----~'AC-AT)

= S-K(1

Both M and N,

i.e. d l / d C

and d l / d T have

negative

(E')

signs.

If the e q u a t i o n s E or E" are r e a l l y universal then for each sand only one set of e x p a n s i o n m e a s u r e m e n t will be s u f f i c i e n t to e v a l u a t e the factor SxK and hence p r e d i c t its m a x i m u m e x p a n s i v i t y in all other conditions. The p h y s i c o - c h e m i c a l reasons for the n e g a t i v e i.e. d l / d C and d l / d T could be the following:

Kallerup sand, 70 C Legend: 1-1N 2-2N S=8aL 0 5 0 ~4 0 (-3 0 Q.. X

s . e ~ ~ : s : ~ _ _ b.- . . . . . . .

10

-

.

.

20

.

30

.

.

40

.

.

50

.

.

.

60

s .

.

70

80

Time in weeks Fig.

5.

Time e x p a n s i o n

curve.

90

signs of M and N

On the basis of our investig a t i o n on a l k a l i - s i l i c a reaction we have r e c e n t l y proposed a new m e c h a n i s m for the a l k a l i - s i l i c a r e a c t i o n and the a s s o c i a t e d e x p a n s i o n (6). A c c o r d i n g to this m e c h a nism during r e a c t i o n Na + and Ca ++ ions together with OHions and water m o l e c u l e s enter the reactive grains; the m a x i m u m entrance is d e t e r m ined by the reactive silica content of the grains. At the same time a part of the reactive silica is set free to move out of the r e a c t i v e grains. The rate of m i g r a t i o n of silica out of the reactive g r a i n s d e p e n d s on the C a ( O H ) 2

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289

ALKALI-SILICA REACTION, EXPANSION MODELING

c o n c e n t r a t i o n of the surrounding liquid phase. The expansion is determined by the rate of net material ingress in the reactive grains i.e. material inflow minus silica outflow. If Ca(OH) 2 concentration in the liquid phase could be lowered without changing other conditions then expansion will also be lowered.

Kallerup sand, 5 0 C Legend: 1-1N 2-2N 0 5 o ~4 0 ¢:3

.2 . . . . .

2--2-

...............

S-Sat.

2" ...........

2

2- .....

,~::.:..s-----.s--s.......s ..............s ................s ........s

._o w

~ o'~ o

..t.~ 10

20

1 30

1 40

50

1 60

1

70

80

90

T i m e in w e e k s

Fig. 6.

Time expansion curve.

Kallerup sand, 3 0 C Legend: 1-1N 2-2N S=SaL o15 o o lo c

.o

w ~5 Q.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

x

IU 0 10

20

30

40

50

60

70

80

90

From the above mechanism one may infer that d l / d T will have a negative sign as the solubility of Ca(OH)2 decreases with increasing temperature (see General and Inorganic Chemistry by Partington McMillan & Co. Ltd. London 1961, p. 370). The coefficient dl/dC could also be negative if the concentration of Ca(OH) 2 decreases with the increasing concentration of sodium chloride. The solubility of Ca(OH) 2 in saturated, 2 and I normal sodium chloride solutions was determined at 20°C. The results are shown in the following table.

T i m e in w e e k s

Fig.

7.

Table

Time expansion curve.

Solubility of Ca(OH) 2 in sodium chloride solution NaCI conc. Satd. 2N IN 0

Kallerup sand, 2 0 C Legend: 1-1N 2-2N S.SaL 015 o o10 c o

.s . ~ : f S - : : : : : ~

c 5 el Q. x

uJ

0 0

10

20

30

40

50

60

70

80

T i m e in w e e k s

Fig. 8.

Time expansion curve.

I

90

CaO (9/1) 0.77 1.55 1.65 1.23

This data is consistent with published data where they o v e r l a p (7). From the above table it can be seen that between IN and tne saturated NaCI solution the solubility of Ca(OH) 2 decreases with increasing concentration of sodium chloride. The results of

290

Vol. 20, No. 2 S. Chatterji and P. Christensen

Table I will e x p l a i n the n e g a t i v e sign of dl/dC. It was a very p l e a s a n t s u r p r i s e that the q u a n t i t a t i v e e x p r e s s i o n s could be rat i o n a l i z e d on the basis of the p r o p o s e d r e a c t i o n m e c h a n i s m (6). The above r e s u l t s could be used d i r e c t l y to p r e d i c t c o n c r e t e expansion. In most c o m m o n c o n c r e t e c o m p o s i t i o n s s a n d - c e m e n t m o r t a r o c c u p i e s about 50% of the total v o l u m e and in the m o r t a r fraction the c e m e n t : s a n d ratio is about 1:3. On the a s s u m p t i o n that a good q u a l i t y n o n - r e a c t i v e a g g r e g a t e has been used as the g r a vel f r a c t i o n then the e x p a n s i v i t y of the c o n c r e t e mix could be c a l c u l a t e d as 0.5 times the e x p a n s i o n of the m o r t a r (8). As the m o r t a r e x p a n s i o n could be c a l c u l a t e d using the e q u a t i o n s E or E" the e x p a n s i v i t y of a c o n c r e t e mix will also be p r e d i c t a b l e . If o t h e r w o r k e r s could r e p r o d u c e the e q u a t i o n E with o t h e r reactive m a t e r i a l s then it will p r o v i d e a r a t i o n a l basis for c h o o s i n g a g g r e g a t e s for a g i v e n e n v i r o n m e n t a l p a r a m e t e r s . Acknowledgement This work was s u p p o r t e d by a g r a n t from the D a n i s h C o u n c i l for I n d u s t r i a l R e s e a r c h (STVF). The a u t h o r s are g r a t e f u l for this grant. References I) S. C h a t t e r j i

- Cement

& Concrete

Res. 8,

647-50,

1978

2) S. C h a t t e r j i

- Cement

& Concrete

Res.

185-88,

1979

3) S. C h a t t e r j i , N. T h a u l o w and A.D. Res. 17, 777-83, 1987 4) A.D.

9,

Jensen

- Cement

& Concrete

Jensen, S. C h a t t e r j i , P. C h r i s t e n s e n and N. T h a u l o w C e m e n t & C o n c r e t e Res. 14, 311-14, 1984

5) J. Bonzel,

J. Krell 1986

and F. Siebel

- Beton,

345-48,

385-89,

6) S. C h a t t e r j i , A.D. Jensen, N. T a h u l o w and P. C h r i s t e n s e n C e m e n t & C o n c r e t e Res. 16, 246-54, 1986 7) W.F.

8) G.

-

-

Linke - " S o l u b i l i t i e s " , Vol I. 4th Edition. D. V a n Nos t r a n d Co. Inc. N e w Y o r k 1958, pp. 636-37

Dahl,

E. P o u l s e n and A. Timm - 6th Inter. Conf. in C o n c r e t e . C o p e n h a g e n , 1983. 249-252.

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